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Big Data Engineering

What is Apache Druid? A Complete Guide

Chandan Gaur | 14 February 2023

What is Apache Druid?

Introduction to Apache Druid

It is a real-time analytics database that is designed for rapid analytics on large datasets. This database is used more often for powering use cases where real-time ingestion, high uptime, and fast query performance is needed.  Druid can be used to analyze billions of rows not only in batch but also in real-time. It offers many integrations with different technologies like Apache Kafka Security, Cloud Storage, S3, Hive, HDFS, DataSketches, Redis, etc. It also follows the immutable past and append-only future. As past events happen once and never change, these are immutable, whereas the only append takes place for new events. It provides users with a fast and deep exploration of large scale transaction data.
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Characteristics of Apache Druid

Some of the exciting characteristics are:

  • Cloud-Native, making easy horizontal scaling
  • Supports SQL for analyzing data
  • REST API enabled for querying or uploading data

What are its use cases?

Some of the common use cases of Druid are:
  • Clickstream analytics
  • Server metrics storage
  • OLAP/Business intelligence
  • Digital Marketing/Advertising analytics
  • Network Telemetry analytics
  • Supply chain analytics
  • Application performance metrics

What are its key features?

Druid’s core architecture is made by combining the ideas of different data warehouses, log search systems, and time-series databases.

Columnar Storage Format

It uses column-oriented storage; hence only loads required columns needed for a particular query. It helps in fast scans and aggregations.

Parallel Processing

It can process a query in parallel across the entire cluster. It is also termed as Massively Parallel Processing.

Scalable Distributed System

Druid is mostly deployed in clusters ranging from tens to hundreds that offer ingest rate of millions of records/sec, query latencies of sub-second to a few seconds, and retention of trillions of records.

Real-time or Batch Ingestion

Druid can ingest data either in real-time (Ingested data can be queried immediately) or in batches.


It is a fault-tolerant architecture that won’t lose data. Once Druid ingests data, its copy is safely stored in deep storage (Cloud Storage, Amazon S3, Redis, HDFS, many more). Users' data can be easily recovered from this deep storage even if all the Druid’s servers fail. This replication ensures that queries are still possible while the system recovers.


Druid uses concise and roaring compressed bitmap indexes to create indexes that help in faster filtering.

Timestamp Partitioning

Every data in Druid must have a timestamp column as the data is always partitioned by time, and every query has a time filter.

Easy Integration with Existing Pipelines

Users can easily stream data natively using Druid from message buses like Kafka, kinesis, and many more. It can also load batch files from the data lakes like HDFS and Amazon S3.

General Architecture of Apache Druid

Druid is mainly composed of the following processes:

  • Coordinator – This process manages data availability on the cluster.
  • Overlord – This process controls the assignment of data ingestion workloads.
  • Broker – This helps handle queries from external clients.
  • Historical – This process store data that is queryable.
  • Middle manager – This process is responsible for ingesting the data.
Router – These processes are used to route requests to brokers, Coordinators, and Overlords. These processes are optional.

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apache-druid-architecture The processes described above are organized into 3 types of servers: Master, Query, and Data.


It runs the Coordinator and Overlord. Basically, it manages big data ingestion and availability. Master is responsible for the ingestion of jobs and coordinating the availability of data on the “Data Servers”.


It runs Brokers and Optional Router processes. Basically, it handles queries and external clients by providing the endpoints of applications that users and clients interact with, routing queries to Data servers or other Query servers.


It runs Middle Managers and Historical processes. This helps execute jobs and store the queryable data. Other than these 3 servers and six processes, Druid also requires storage for Metadata and Deep Storage.

Metadata Storage

It is basically used to store the metadata of the system (Audit, Datasource, Schemas, and so on). For experimental purposes, the environment suggested using Apache Derby. Derby is the default metadata store for Druid, but it is not suitable for production. For production purposes, MySQL or PostgreSQL is the best choice. Metadata storage stores the entire metadata, which is very useful for the cluster of Druid to work. Derby is not used for production as it does not support a multi-node cluster with high availability. MySQL as a metadata storage database is used to acquire:
  • Long term flexibility
  • Scaling on budget
  • Good with large datasets
  • Good high read speed
PostgreSQL, as a metadata storage database, is used to acquire:
  • Complex database designs
  • Performing customized procedures
  • Diverse indexing technique
  • Variety of replication methods
  • High read and write speed.

Deep Storage

Apache Druid uses separate storage for any data ingested that makes it fault-tolerant. Some of Deep Storage Technologies are Cloud Storage, Amazon S3, HDFS, Redis, and many more.
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Data Ingestion in Druid

Data in Druid is organized into segments that generally have rows up to a few million. Loading data in Druid is known as Ingestion or Indexing. Druid fully supports batch ingestion and streaming ingestion. Some of the technologies supported by Druid is Kinesis, Cloud Storage, Apache Kafka, and local storage. Druid requires some structure to the data it ingests. In general, data should consist of OS timestamp, metrics and dimensions.

Zookeeper for Apache Druid

Ittransf uses Apache Zookeeper to integrate all the solutions. Users can use Zookeeper that comes with Druid for experiments, but one has to install Zookeeper for production. It’s cluster can only be as stable by a Zookeeper. Zookeeper is responsible for most of the communications that keep the Druid cluster functioning as Druid nodes are prevented from talking to each other. apache-druid-zookeeper

Duties of a Zookeeper

Zookeeper is responsible for the following operations:
  • Segment “publishing” protocol from Historical
  • Coordinator leader election
  • Overlord and MiddleManager task management
  • Segment load/drop protocol between Coordinator and Historical
  • Overlord leader election

How to Keep a Zookeeper Stable?

For maximum Zookeeper stability, the user has to follow the following practices:
  • There should be a Zookeeper dedicated to Druid; avoid sharing it with any other products/applications.
  • Maintain an odd number of Zookeepers for increased reliability.
  • For highly available Zookeeper, 3-5 Zookeeper nodes are recommended. Users can either install Zookeeper on their own system or run 3 or 5 master servers and configure Zookeeper on them appropriately.
  • Share Zookeeper’s location with a master server rather than doing so with data or query servers. This is done because query and data are far much work-intensive than the master node (coordinator and overlord).
  • To fully achieve high availability, it is recommended to never out Zookeeper behind a load balancer.
If Zookeeper goes down, the cluster will operate. Failing of Zookeeper would neither result in addition to new data segments nor can it effectively react to the loss of one of the nodes. So, the failure of Zookeeper is a degraded state.
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How to monitor Apache Druid?

Users can monitor Druid by using the metrics it generates. Druid generates metrics related to queries, coordination and ingestion. These metrics are emitted as a JSON object. It is either emitted to a runtime log file or over HTTP (to service like Kafka). The emission of a metric is disabled by default.

Fields of Metrics Emitted

Metrics emitted by Druid share a common set of fields.
  • Timestamp – the time at which metric was created
  • Metric – the name given to the metric
  • Service – the name of the service that emitted the metric
  • Host – the name of the host that emitted the metric
  • Value – the numeric value that is associated with the metric emitted

Briefing About Available Metrics

Metric emitted may have dimensions beyond the one listed. To change the emission period of Druid that is 1 minute by default, one can use `druid.monitoring.emissionPeriod` to change the default value. Metrics available are:
  • Query Metrics, mainly categorized as Broker, Historical, Real-time, Jetty and Cache
  • SQL Metrics (Only if SQL is enabled)
  • Ingestion Metrics (Kafka Indexing Service)
  • Real-time Metrics (Real-time process, available if Real-time Metrics Monitor is included)
  • Indexing Service
  • Coordination
  • JVM (Available if JVM Monitor module is included)
  • Event Receiver Firehose (available if Event Receiver Firehose Monitor module is included)
  • Sys (Available if Sys Monitor module is included)
  • General Health, mainly Historical
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Apache Druid is the best in the market when it comes to analyzing data in clusters and providing brief insight to all the data processed. Plus having Zookeeper by the side, one can ease up their working with it and rule the DataOps market. Also, there are many libraries to interact with it. To Validate the running of service, one can use JPS commands. As Druid nodes are java processes, they would show up when JPS commands '$ jps -m' are used. With that much ease in monitoring Druid and working with such a vast architecture of Druid, it is really the last bite of an ice-cream for a DataOps Engineer.